Dear Lisa:

The Scripture passage in question Colossians 1:24 reads:

Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the church...

The Navarre Bible Commentary, one of the best Catholic commentaries published, explains:

Jesus Christ our Lord perfectly accomplished the work the Father gave him to do (cf. John 17:4); as he said himself when he was about to die, "It is finished", it is accomplished (John 19:30).

From that point onwards objective redemption is an accomplished fact. All men have been saved by the redemptive death of Christ. However, St. Paul says that he complete in his flesh "what is lacking in Christ's afflictions"; what does he mean by this? The most common explanation of this statement is summarized by St. Alphonsus as follows:

Can it be that Christ's passion alone was insufficient to save us? It left nothing more to be done, it was entirely sufficient to save all men. However, for the merits of the Passion to be applied to us, according to St. Thomas (Summa theologiae, III, q. 49, a. 3), we need to cooperate (subjective redemption) by patiently bearing the trials God sends us, so as to become like our head, Christ" (St. Alphonsus, Thoughts on the Passion, 10).

St. Paul is applying this truth to himself. Jesus Christ worked and strove in all kinds of ways to communicate his message of salvation, and then he accomplished the redemption by dying on the Cross. The Apostle is mindful of the Master's teaching and so he follows in his footsteps (cf. 1 Peter 2:21), takes up his cross (cf. Matthew 10:38) and continues the task of bringing Christ's teaching to all men.

Faith in the fact that we are sharing in the sufferings of Christ, John Paul II says, gives a person "the certainty that in the spiritual dimension of the work of a Redemption he is serving, like Christ, the salvation of his brothers and sisters. Therefore he is carrying out an irreplaceable service. In the Body of Christ, which is ceaselessly born of the Cross of the Redeemer, it is precisely suffering permeated by the spirit of Christ's sacrifice that is the irreplaceable mediator and author of the good things which are indispensable for the world's salvation. It is suffering, more than anything else, which clears the way for the grace which transforms human souls. Suffering, more than anything else, makes present in the history of humanity the force of the Redemption" (Salvifici doloris, 27). [my emphasis]

The Ignatius Bible Study Bible explains it this way:

[When St. Paul refers to "what is lacking" he is referring to] the suffering that remains for believers in the trials of life. Suffering is a mission for all the faithful as a means of conforming ourselves to Christ (Romans 8:17); Philippians 3:10)...

These words could be misunderstood to mean that the suffering of Christ was not sufficient for redemption and that the suffering of the saints must be added to complete it. This, however, would be heretical. Christ and his Church are one mystical person, and while the merits of Christ, the head, are infinite, the saints acquire merit in a limited degree. What is "lacking", then, pertains to the afflictions of the entire Church, to which Paul adds his own amount (St. Thomas Aquinas, Commentary on Colossians 1, 6)

To summarize: Christ's suffering on the Cross for the redemption of mankind is complete. What is lacking is for us to add our own suffering to the Cross; to share in the suffering on the Cross by lifting up and joining our suffering with that of Christ's.

As for the celebration of the Sacrifice of the Holy Mass, it is during Mass that we can profoundly offer our own suffering to Christ, to join Christ in his suffering.

The Mass is a re-presentation of the Sacrifice of Christ on the Cross. It is not a symbol, nor is it crucifying Christ again as anti-Catholics suggest. The Mass is the self-same sacrifice of Christ 2000 years ago. It is a living continuation of the sacrifice Christ made 2000 years ago.

Vatican II said it this way:

"At the Last Supper, on the night he was betrayed, our Savior instituted the Eucharistic Sacrifice of his Body and Blood. He did this in order to perpetuate the sacrifice of the cross throughout the centuries until he should come again, and so to entrust to his beloved spouse, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a paschal banquet in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us" (Sacrosanctum Concilium 47).

John A. O’Brien, writing in The Faith of Millions, explains further:

"The manner in which the sacrifices are offered is alone different: On the cross Christ really shed his blood and was really slain; in the Mass, however, there is no real shedding of blood, no real death; but the separate consecration of the bread and of the wine symbolizes the separation of the body and blood of Christ and thus symbolizes his death upon the cross. The Mass is the renewal and perpetuation of the sacrifice of the cross in the sense that it offers [Jesus] anew to God . . . and thus commemorates the sacrifice of the cross, reenacts it symbolically and mystically, and applies the fruits of Christ’s death upon the cross to individual human souls. All the efficacy of the Mass is derived, therefore, from the sacrifice of Calvary."

Christ does not die again in the Mass. His death was once and for all time. But, Christ has made himself a "living sacrifice" that we celebrate in the Mass until the Second Coming.

Because the Mass is the re-presentation of the living Sacrifice of Christ, this time more than any other is a time to offer up our own sufferings to Christ, joining our sufferings with His and thereby making our own sufferings redemptive.

I hope all this helps to understand. This is a more complicated theology.

God Bless,
Bro. Ignatius Mary